Mask blank substrate, mask blank, photomask, and methods of manufacturing the same

ABSTRACT

A mask blank substrate for a photomask is chucked on a mask stage of an exposure apparatus. A main surface, on the side where a thin film for a transfer pattern is to be formed, of the mask blank substrate has a flatness of 0.3 μm or less in a 142 mm square area including its central portion and has a convex shape being relatively high at its central portion and relatively low at its peripheral portion. The difference upon fitting, to the main surface of the mask blank substrate, a virtual reference main surface, having a spherical shape in a 132 mm square area, of a virtual reference substrate is 40 nm or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. application Ser.No. 12/536,966 filed Aug. 6, 2009, which is based on and claims priorityunder 35 U.S.C. §119 to Japanese Patent Application No. 2008-204164,filed on Aug. 7, 2008, the disclosure of which is incorporated herein inits entirety by reference.

TECHNICAL FIELD

This invention relates to a mask blank substrate and a mask blank for aphotomask for use in a photolithography process.

BACKGROUND ART

In a photolithography process of semiconductor manufacturing processes,a photomask is used. Following the miniaturization of semiconductordevices, a demand for miniaturization in this photolithography processhas been increasing. Particularly, an increase in NA of an exposureapparatus using ArF exposure light (193 nm) has proceeded for adaptationto the miniaturization and a further increase in NA is proceedingfollowing the introduction of the immersion exposure technique. Foradaptation to the demand for the miniaturization and the increase in NAdescribed above, it is required to enhance the flatness of a photomask.That is, in view of the fact that the allowable amount of positionoffset of a transfer pattern due to the flatness has been reducedfollowing the reduction in pattern line width and that the focuslatitude in the photolithography process has been reduced following theincrease in NA, the flatness of main surfaces of a mask substrate,particularly the main surface on the side where a pattern is to beformed (hereinafter, the main surface on this side will be referred tosimply as a “main surface” or a “substrate main surface”), is becomingmore important.

On the other hand, when the photomask is chucked on a mask stage of anexposure apparatus by a vacuum chuck, it may happen that the photomaskis largely deformed upon chucking due to the affinity with the maskstage or the vacuum chuck. That is, since the product management isconducted in terms of the flatness of the photomask before chucking, itmay happen that even if the shape of the main surface thereof beforechucking has high flatness, when the photomask is chucked on the maskstage of the exposure apparatus, it is deformed to significantly degradethe flatness thereof depending on the affinity with the mask stage orthe vacuum chuck. This tendency is outstanding particularly in the caseof a substrate that tends to be distorted due to low symmetry of theshape of its main surface. Thus, it is becoming necessary to considerthe flatness of the photomask when it is chucked by the vacuum chuck.There has been proposed a method of selecting a mask substrate havingexcellent flatness after chucking on a mask stage of an exposureapparatus (see JP-A-2003-50458 (Patent Document 1)).

DISCLOSURE OF THE INVENTION

According to the conventional method, however, for each of masksubstrates (mask blank substrates), it is necessary to obtaininformation indicating the surface shape of a main surface andinformation indicating the flatness of a main surface before and afterchucking on a mask stage of an exposure apparatus or to obtain, based onthe flatness of a main surface and the structure of a mask chuck,information indicating the flatness of the main surface by simulationwhen the mask substrate is set in an exposure apparatus. Therefore,conventionally, it takes much time and labor to select a mask substratehaving excellent flatness after chucking on the mask stage of theexposure apparatus. Further, the structure for chucking a mask substrateon a mask stage differs depending on an exposure apparatus and thus itis necessary to select a mask substrate per exposure apparatus.

Conventionally, use is made of a technique of aiming at finishingsubstrate main surfaces to have higher flatness in a substrate polishingprocess, selecting substrates polished to high flatness from amongpolished substrates, and extracting those substrates by simulation thatmatch an exposure apparatus to be used. However, when the substrates arepolished so as to have high flatness by the use of a double-sidepolishing machine adapted to simultaneously polish a plurality ofsubstrates, the number of substrates reaching a target flatness is smallamong the simultaneously polished substrates and thus the yield ofsubstrate production is low, which has been a problem. Further, asdescribed above, the substrate polished to high flatness is notnecessarily fit for the exposure apparatus to be used and thus the yieldof substrate production is significantly reduced, which has been aproblem.

This invention has been made in view of these circumstances and has anobject to provide a mask blank substrate, a mask blank, and a photomaskthat do not require a simulation for the flatness of a mask substrateafter chucking on a mask stage of an exposure apparatus and that canrealize a desired flatness after chucking regardless of the chuckstructure of an exposure apparatus.

According to this invention, there is provided a mask blank substratefor a photomask to be chucked on a mask stage of an exposure apparatus,

wherein a main surface, on a side where a thin film for a transferpattern is to be formed, of the mask blank substrate has a flatness of0.3 μm or less in a 142 mm square area including its central portion andhas a convex shape being relatively high at its central portion andrelatively low at its peripheral portion, and a difference upon fitting,to the main surface, a virtual reference main surface, having aspherical shape in a 132 mm square area, of a virtual referencesubstrate is 40 nm or less.

In the mask blank substrate according to this invention, the virtualreference main surface preferably has a flatness of 0.2 μm or less inthe 132 mm square area.

A mask blank according to this invention comprises the aforementionedmask blank substrate and a light-shielding film formed on the mainsurface of the mask blank substrate.

In the mask blank according to this invention, the light-shielding filmis preferably made of a material mainly composed of chromium or amaterial mainly composed of molybdenum silicide.

A photomask according to this invention has a transfer pattern formed bythe light-shielding film of the aforementioned mask blank.

According to this invention, there is provided a mask blank substratemanufacturing method comprising the steps of:

polishing a main surface, on a side where a thin film is to be formed,of a substrate to have a flatness of 0.3 μm or less in a 142 mm squarearea including its central portion;

measuring a surface shape of the main surface of the substrate in a 132mm square area thereof,

performing fitting between the measured surface shape of the substrateand a virtual reference main surface of a virtual reference substrate,and

selecting as a mask blank substrate the substrate if a differencetherebetween is 40 nm or less.

A mask blank manufacturing method according to this invention comprisesa step of forming a light-shielding film on a main surface, on a sidewhere a thin film is to be formed, of a mask blank substrate obtained bythe aforementioned method.

A photomask manufacturing method according to this invention comprises astep of forming a transfer pattern in a light-shielding film of a maskblank obtained by the aforementioned method.

For a mask blank substrate of this invention, a selection is made, inadvance, of a virtual reference substrate in which the shape of avirtual reference main surface is a convex shape being relatively highat its central portion and relatively low at its peripheral portion andis a spherical shape in a 132 mm square area of the virtual referencemain surface, i.e. the substrate having the ideal main surface shape.Then, fitting is performed with respect to the virtual reference mainsurface in a 132 mm square area, including a central portion, of a mainsurface, on the side where a thin film is to be formed, of an actuallymanufactured substrate. If the difference therebetween is 40 nm or lessand the flatness in a 142 mm square area, including the central portion,of the main surface of the actually manufactured substrate is 0.3 μm orless, the actually manufactured substrate is judged to be a passed maskblank substrate. Therefore, it is not necessary to simulate the flatnessof a mask substrate after chucking it on a mask stage of an exposureapparatus.

Polishing accuracy required when manufacturing the mask blank substrateis relaxed as compared with that required when manufacturing a substratewith very high flatness and, further, even a substrate that originallycould not satisfy very high flatness can be shipped as a passed productthat can fabricate a photomask adapted to exhibit sufficient transferperformance in a predetermined exposure apparatus. Thus, it is possibleto significantly improve the yield. Much time is required for simulatingmanufactured substrates one by one, but in the case of this invention,it is only required to carry out fitting of manufactured substrates tothe virtual reference main surface of the virtual reference substratedetermined in advance and thus the time required for judging thesubstrates can be significantly reduced. Further, by setting the virtualreference main surface of the virtual reference substrate that can becommonly used for various chuck-type exposure apparatuses, it ispossible to provide a mask blank substrate that can fabricate aphotomask adapted to exhibit predetermined transfer performanceregardless of the chuck structure of an exposure apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view, as seen in a direction of a substrate mainsurface, when a photomask is placed on chuck stages of an exposureapparatus;

FIG. 2A is a diagram showing the shape of the photomask before chuckingit on the chuck stages and is a side view as seen in a direction Aindicated in FIG. 1;

FIG. 2B is a diagram showing the shape of the photomask before chuckingit on the chuck stages and is a side view as seen in a direction Bindicated in FIG. 1;

FIG. 3A is a diagram showing the shape of the photomask after chuckingit on the chuck stages and is a side view as seen in the direction Aindicated in FIG. 1;

FIG. 3B is a diagram showing the shape of the photomask after chuckingit on the chuck stages and is a side view as seen in the direction Bindicated in FIG. 1;

FIG. 4A is a contour diagram showing the shape of a main surface of asubstrate to which this invention is applied, wherein the shape of themain surface of the substrate before chucking on chuck stages of anexposure apparatus is illustrated;

FIG. 4B is a contour diagram showing the shape of the main surface ofthe substrate to which this invention is applied, wherein the shape ofthe main surface of the substrate after chucking on the chuck stages ofthe exposure apparatus is illustrated;

FIG. 5A is a plan view, as seen in a direction of a main surface, of amask blank substrate according to an embodiment of this invention;

FIG. 5B is a sectional view taken along line Y1-Y1 in FIG. 5A;

FIG. 5C is a sectional view taken along line XY1-XY1 in FIG. 5A;

FIG. 6 is a diagram showing a partial enlarged section of the mask blanksubstrate shown in FIG. 5B;

FIG. 7 is a diagram showing a schematic structure of a sputteringapparatus for use in manufacturing a mask blank according to theembodiment of this invention;

FIG. 8 is a contour diagram showing the shape of a main surface of aglass substrate manufactured in Example 2;

FIG. 9 is a diagram showing the shapes of the main surface in sectionsalong line XYR1-XYR1 and line XYR2-XYR2 of the glass substrate shown inFIG. 8;

FIG. 10 is a contour diagram showing the shape of a virtual referencemain surface;

FIG. 11 is a diagram upon fitting the virtual reference main surfaceshown in FIG. 10 to the glass substrate shown in FIG. 8;

FIG. 12 is a diagram showing fitting differences upon performing thefitting in FIG. 11;

FIG. 13A is a schematic front sectional view for explaining a processingstate by an MRF processing method in Example 7; and

FIG. 13B is a schematic side sectional view for explaining theprocessing state by the MRF processing method in Example 7.

DETAILED DESCRIPTION OF THE INVENTION

With respect to a mask blank substrate of this invention, rather thanattaching importance to manufacturing a substrate having a main surfacewith very high flatness when it is not chucked on a mask stage,importance is attached to causing a main surface, where a transferpattern of a photomask is to be formed, of a substrate, when it ischucked on a mask stage, to be flat enough to exhibit sufficienttransfer performance in transfer of the transfer pattern by an exposureapparatus.

As a result of analyzing a change in shape of a substrate when aphotomask is chucked on a mask stage of an exposure apparatus, thefollowing have been found. Normally, when chucking a photomask on a maskstage of an exposure apparatus, portions of a main surface on twoopposite end face sides of the photomask are used as chuck areas.

Generally, a substrate with a main surface polished by a polishingmachine tends to have, on the nature of the polishing, a sectional shapein which, basically, the center is high and the end face sides are low,and thus a photomask fabricated from the substrate having such a mainsurface shape also has the same surface shape and is chucked on a maskstage of an exposure apparatus. FIG. 1 is a plan view when a photomaskhaving such a shape is placed on chuck stages (portions, with which asurface of a photomask is brought into direct contact so as to bechucked, of a mask stage) of an exposure apparatus. FIG. 2A is a sideview, as seen in a direction A (chuck stage short-side direction)indicated in FIG. 1, showing a state before the photomask is chucked onthe chuck stages. FIG. 2B is a side view, as seen in a direction B(chuck stage long-side direction) indicated in FIG. 1, also showing thestate before the photomask is chucked on the chuck stages. As seen fromFIG. 2A, both end face sides of the photomask are warped up on the chuckstage short-side sides due to the surface shape of the photomask. Asseen from FIG. 2B, both end face sides of the photomask are warped up onthe chuck stage long-side sides due to the surface shape of thephotomask.

In such a placed state, when the photomask is chucked on the chuckstages, as shown in FIGS. 3A and 3B, the warped-up four end face sidesof the photomask are pulled by suction and thus a force having afunction of deforming the photomask upward into quadratic componentsfrom the four end face directions is applied to the substrate. That is,a force adapted to deform a main surface of a substrate into a quadraticsurface (spherical shape) so as to be convex upward toward the centerfrom chuck areas on the four end face sides tends to be applied to thesubstrate.

FIGS. 4A and 4B are diagrams showing the shapes of a substrate accordingto this invention before (before suction) and after (after suction)chucking the substrate on a mask stage of an exposure apparatus, whereinFIG. 4A is a diagram showing the shape of the substrate before suctionand FIG. 4B is a diagram showing the shape of the substrate aftersuction. As seen from FIG. 4A, four corners of a main surface of thesubstrate are a little higher than chuck areas of the main surface andthe main surface gradually increases in height toward its center. Thatis, generally circular contour lines are shown in the substrate beforesuction. In the substrate after suction, as seen from FIG. 4B, generallyrectangular contour lines are shown wherein the number of contour linesin a 132 mm square is small and the intervals thereof are long. That is,the shape of the substrate main surface after chucking is significantlyimproved in flatness as compared with that before chucking.

Taking this tendency into account, an ideal substrate (virtual referencesubstrate) is assumed for a mask blank substrate of this invention suchthat the shape of its main surface (virtual reference main surface) is aconvex shape being relatively high at its central portion and relativelylow at its peripheral portion and is a spherical shape having a heightdifference of at least 0.3 μm or less in a 132 mm square area of thevirtual reference substrate. As a result of simulating the shape of thevirtual reference main surface after chucking a photomask using thisvirtual reference substrate in an exposure apparatus, the flatness ofthe virtual reference main surface is 0.08 μm or less. Fitting isperformed with respect to the reference curved surface shape of thevirtual reference substrate in a 132 mm square area, including a centralportion, of a main surface, on the side where a thin film is to beformed, of a substrate actually manufactured by carrying outpredetermined polishing. If the difference therebetween is 40 nm or lessand the flatness in a 142 mm square area, including the central portion,of the main surface of the actually manufactured substrate is 0.3 μm orless, the actually manufactured substrate is judged to be a passed maskblank substrate. The mask blank substrate thus obtained can satisfy theflatness required for a photomask of DRAM half-pitch (hp) 32 nmgeneration in a 132 mm square area being an area where a transferpattern is formed.

When fitting the virtual reference main surface to the 132 mm squarearea in the main surface of the actually manufactured substrate (actualsubstrate) after polishing, it is preferable to perform the fitting in aheight relationship in which the virtual reference main surface is atleast higher than the main surface of the actual substrate at theboundary of the 132 mm square area. It is more preferable to perform thefitting in a height relationship in which the virtual reference mainsurface coincides in height as much as possible with the main surface ofthe actual substrate at the boundary of the 132 mm square area.

The spherical shape of the virtual reference main surface referred toherein is not limited to a partial shape of a complete sphericalsurface. Depending on a tendency of the sectional shape of an actualsubstrate after polishing due to characteristics of a polishing machineused in a polishing process and an suction force of a chuck at a maskstage of an exposure apparatus in which the actual substrate is used,there is a case where a tendency increases that a strong deformationforce is applied more to a certain pair of end face sides of thesubstrate than to the other pair of end face sides perpendicular to thecertain pair of end face sides. In such a case, the shape of the virtualreference main surface may be an elliptical spherical shape.

By applying this invention, polishing accuracy required whenmanufacturing the mask blank substrate is relaxed as compared with thatrequired when manufacturing a substrate with very high flatness and,further, even a substrate that originally could not satisfy very highflatness can be shipped as a passed product that can fabricate aphotomask adapted to exhibit sufficient transfer performance in apredetermined exposure apparatus. Thus, it is possible to significantlyimprove the yield. Much time is required for simulating manufacturedsubstrates one by one, but in the case of this invention, it is onlyrequired to carry out fitting of manufactured substrates to the virtualreference main surface of the virtual reference substrate determined inadvance and thus the time required for judging the substrates can besignificantly reduced. Further, by selecting an ideal virtual referencemain surface of a virtual reference substrate per chuck type of anexposure apparatus and then setting a commonly applicable virtualreference main surface of a virtual reference substrate based on thosereference curved surfaces, it is possible to provide a mask blanksubstrate that can fabricate a photomask adapted to exhibitpredetermined transfer performance regardless of the chuck structure ofan exposure apparatus.

The reason for allowing, as a passed product, only the manufacturedsubstrate having a flatness of 0.3 μm or less in the 142 mm square area,including the central portion, of the main surface thereof is that, inthe case of a photomask having a flatness greater than 0.3 μm, thedeformation amount upon chucking on an exposure apparatus is large sothat the position offset in a plane direction of a transfer patternformed on the photomask becomes large.

An area, where a transfer pattern is formed, of a photomask is within132 mm×104 mm. However, since the deformation of a substrate is causedby chucking of the photomask by an exposure apparatus, it is necessaryto also take into account an area outside the area where the transferpattern is to be formed. If the flatness of the outside area is poor,there is a possibility that the deformation amount of the substrateafter chucking is large. If the deformation amount of the substrate islarge, the moving amount of the transfer pattern formed on a mainsurface of the substrate is large so that the pattern position accuracyis lowered. Taking them into account, the flatness measurement area inthe actual substrate is set to be the 142 mm square area.

The manufactured substrate is judged to be a passed product if thedifference upon performing fitting with respect to the reference curvedsurface shape of the virtual reference substrate in the 132 mm squarearea, including the central portion, of the main surface of themanufactured substrate is 40 nm or less. This difference upon performingthe fitting is such that, upon fitting the main surface of themanufactured substrate to the reference curved surface, a maximum of 40nm is allowed for the difference when the main surface of themanufactured substrate is located above the reference curved surface anda maximum of 40 nm is allowed for the difference when the main surfaceof the manufactured substrate is located below the reference curvedsurface.

Hereinbelow, an embodiment of this invention will be described in detailwith reference to the accompanying drawings.

FIGS. 5A to 5C are diagrams for explaining a mask blank substrate 1according to the embodiment of this invention, wherein FIG. 5A is a planview, FIG. 5B is a sectional view taken along line Y1-Y1 in FIG. 5A, andFIG. 5C is a sectional view taken along line XY1-XY1 in FIG. 5A. Theshape shown in FIG. 5B is substantially the same as the shape in asectional view taken along line X1-X1 in FIG. 5A and the shape shown inFIG. 5C is substantially the same as the shape in a sectional view takenalong line XY2-XY2 in FIG. 5A.

In the mask blank substrate 1 shown in FIG. 5A, a main surface 2 on theside where a thin film for a transfer pattern is to be formed has aflatness of 0.3 μm or less in a 142 mm square area including its centralportion and has a convex shape being relatively high at its centralportion and relatively low at its peripheral portion. In FIG. 5A, it isgiven that the length of one side of the mask blank substrate 1 is Ls(A=152 mm), the length of one side of a 142 mm square area is Lb (B=142mm), and the length of one side of a 132 mm square area is Lp (C=132mm). The flatness in the 142 mm square area represents a difference Hbetween the highest portion and the lowest portion of the mask blanksubstrate 1 in that area as shown in FIGS. 5B and 5C.

In the mask blank substrate 1, the difference upon fitting apredetermined virtual reference main surface of a virtual referencesubstrate to the shape of the main surface 2 is 40 nm or less. Herein,the virtual reference substrate is such that the shape of the virtualreference main surface is a convex shape being relatively high at itscentral portion and relatively low at its peripheral portion and is aspherical shape in a 132 mm square area of the virtual reference mainsurface. More specifically, the virtual reference substrate is such thatthe virtual reference main surface has a flatness of 0.3 μm or less,preferably 0.2 μm or less in the 132 mm square area including itscentral portion. Particularly in a virtual reference substrate forobtaining a mask blank substrate that can be commonly used for variouschuck-type exposure apparatuses, it is preferable that a virtualreference main surface has a shape defined by a true spherical surface.

FIG. 6 is a partial enlarged sectional view of the mask blank substrate1 shown in FIG. 5B. A virtual reference main surface 3 is a main surfaceof a virtual reference substrate and FIG. 6 shows a state where thevirtual reference main surface 3 is fitted to the main surface 2. InFIG. 6, D₁ and D₂ represent differences upon performing fitting to thevirtual reference main surface 3 in the 132 mm square area (areaindicated by Lp in FIG. 5A), including the central portion, of the mainsurface 2. D₁ represents a maximum difference (absolute value) amongdifferences at portions where the main surface 2 is located above thevirtual reference main surface 3, while, D₂ represents a maximumdifference (absolute value) among differences at portions where the mainsurface 2 is located below the virtual reference main surface 3. Ofthese differences D₁ and D₂, the larger difference is 40 nm or less.

The above-mentioned measurement area is preferably the 132 mm squarearea, including the central portion, of the mask blank substrate 1. Byensuring the flatness in this area, transfer of a fine pattern can beaccurately carried out.

The shape of the main surface 2 of the mask blank substrate 1 wasmeasured by a wavelength-shift interferometer using a wavelengthmodulation laser. This wavelength-shift interferometer calculatesdifferences in height of a measuring surface of a mask blank substratefrom interference fringes generated by the interference betweenreflected light reflected from the measuring surface and a back surfaceof the mask blank substrate and a measuring apparatus reference surface(front reference surface), detects differences in frequency of theinterference fringes, and separates the interference fringes, therebymeasuring the shape of irregularities of the measuring surface.

In this invention, a glass substrate can be used as the mask blanksubstrate. The glass substrate is not particularly limited as long as itcan be used for a mask blank. For example, as a material of the glasssubstrate, there can be cited a synthetic quartz glass, a soda-limeglass, an aluminosilicate glass, a borosilicate glass, an alkali-freeglass, or the like. In the case of a glass substrate of an EUV maskblank, in order to suppress the distortion of a transfer pattern due toheat in exposure, use is made of a glass material having a low thermalexpansion coefficient in a range of about 0±1.0×10⁻⁷/° C., preferably ina range of about 0±0.3×10⁻⁷/° C. Further, since the EUV mask blank isformed with many films on the glass substrate, use is made of ahigh-rigidity glass material that can suppress deformation due to filmstress. Particularly, a glass material having a high Young's modulus of65 GPa or more. For example, use is made of an amorphous glass such as aSiO₂—TiO₂-based glass or a synthetic quartz glass or a crystallizedglass in which a β-quartz solid solution is precipitated.

Such a mask blank substrate can be manufactured through, for example, arough polishing process, a precision polishing process, and anultra-precision polishing process.

A substrate to be manufactured is polished so as to be fitted to theshape of the virtual reference main surface of the virtual referencesubstrate. When the shape of the reference curved surface to be fittedto is, for example, a spherical surface, the shape of the substrate isadjusted in the respective polishing processes so as to approximate acurved surface defined by x²+y²+z²=r² (r: radius of curvature). Thecurved surface shape of the virtual reference main surface having aflatness of 0.3 μm or less in the 132 mm square area is such that itsradius of curvature r is about 14,500,000 mm or more, and the curvedsurface shape of the virtual reference main surface having a flatness of0.2 μm or less in the 132 mm square area is such that its radius ofcurvature r is about 21,720,000 mm or more.

By forming at least a light-shielding film on the main surface, havingthe above-mentioned convex shape, of the mask blank substrate describedabove, a mask blank can be obtained. As a material of thislight-shielding film, chromium or molybdenum silicide can be cited. Inthe case of a chromium-based light-shielding film, nitrogen, oxygen,and/or carbon may be added to Cr. In the case of a molybdenumsilicide-based light-shielding film, nitrogen, oxygen, and/or carbon maybe added to MoSi.

Depending on the use and structure of a photomask, another film such asan antireflection film or a semi-transmissive film may be appropriatelyformed. As a material of the antireflection film, it is preferable touse MoSiON, MoSiO, MoSiN, MoSiOC, MoSiOCN, or the like. As a material ofthe semi-transmissive film, it is preferable to use CrO, CrON, MoSiN,MoSiON, or the like.

Further, an etching mask film having etching resistance to thelight-shielding film or the antireflection film may be formed on such afilm and an etching stopper film may be formed between the substrate andthe light-shielding film.

The light-shielding film can be formed by sputtering. As a sputteringapparatus, it is possible to use a DC magnetron sputtering apparatus, anRF magnetron sputtering apparatus, or the like. When sputtering thelight-shielding film on the mask blank substrate, it is preferable torotate the substrate and to dispose a sputtering target at a positioninclined by a predetermined angle with respect to a rotational axis ofthe substrate, thereby forming the light-shielding film. By such a filmforming method, it is possible to minimize in-plane variation of thelight-shielding film and thus to uniformly form the light-shieldingfilm. Since the mask blank substrate of this invention is such that themain surface has a convex shape describing a predetermined curvedsurface (e.g. quadratic surface) from the substrate center as a vertextoward the outside, and thus is a highly symmetric substrate, it ispossible to obtain a mask blank with high in-plane symmetry by formingthe light-shielding film in the manner described above.

In the case of carrying out the film formation by rotating the substrateand disposing the sputtering target at the position inclined by thepredetermined angle with respect to the rotational axis of thesubstrate, the in-plane distributions of the phase angle and thetransmittance also change by the positional relationship between thesubstrate and the target. The positional relationship between thesubstrate and the target will be explained with reference to FIG. 7. Theoffset distance (distance between the central axis of the substrate anda straight line passing through the center of the target and parallel tothe central axis of the substrate) is adjusted by an area in which thephase angle and transmittance distributions are to be ensured.Generally, when such an area is large, the required offset distancebecomes long. In this embodiment, in order to realize a phase angledistribution±2° or less and a transmittance distribution±0.2° or less inthe substrate in the 142 mm square area, the offset distance is requiredto be about 200 mm to about 350 mm and is preferably 240 mm to 280 mm.The optimal range of the target-substrate vertical distance (T/S)changes depending on the offset distance, but in order to realize aphase angle distribution±2° or less and a transmittancedistribution±0.2° or less in the substrate in the 142 mm square area,the target-substrate vertical distance (T/S) is required to be about 200mm to about 380 mm and is preferably 210 mm to 300 mm. The inclinationangle of the target affects the film forming rate and, in order toobtain a high film forming rate, it is suitably 0° to 45° and preferably10° to 30°.

By patterning at least the light-shielding film by photolithography andetching to form a transfer pattern, a photomask can be manufactured. Anetchant for etching is properly changed depending on a material of afilm to be etched.

Next, a description will be given of Examples carried out for clarifyingthe effect of this invention. In the following Examples, a mask blanksubstrate is a glass substrate.

EXAMPLE 1

The shape of a mask blank substrate to be manufactured in Example 1 issuch that the shape of a virtual reference main surface of a virtualreference substrate is a curved surface shape in the form of a sphericalsurface having a radius of curvature r of 14,508,150 mm and having aflatness of 0.3 μm in a 132 mm square area and a main surface thereof ispolished so as to be fitted to this shape of the virtual reference mainsurface. Specifically, the mask blank substrate is manufactured throughthe following polishing processes.

A predetermined number of glass substrates obtained by lapping andchamfering synthetic quartz glass substrates (152.4 mm×152.4 mm×6.45 mm)were set in a double-side polishing machine and subjected to a roughpolishing process under the following polishing conditions. After therough polishing process, the glass substrates were ultrasonicallycleaned for removing polishing abrasive grains adhering to the glasssubstrates. The polishing conditions such as the processing pressure,the rotational speeds of upper and lower surface plates, and thepolishing time were properly adjusted.

-   -   Polishing Liquid: cerium oxide (average grain size 2 μm to 3        μm)+water    -   Polishing Pad: hard polisher (urethane pad)

Then, a predetermined number of the glass substrates after the roughpolishing were set in a double-side polishing machine and subjected to aprecision polishing process under the following polishing conditions.After the precision polishing process, the glass substrates wereultrasonically cleaned for removing polishing abrasive grains adheringto the glass substrates. The polishing conditions such as the processingpressure, the rotational speeds of upper and lower surface plates, andthe polishing time were properly adjusted. The precision polishing iscarried out by adjusting various conditions so that the shape of a mainsurface, on the side where a transfer pattern is to be formed, of eachglass substrate after the precision polishing process becomes convex atfour corners. This is because the next ultra-precision polishing processhas a feature to preferentially polish four corners of the substratemain surface, and thus this makes it possible to prevent edge exclusionat the four corners and to achieve a flatness of 0.3 μm or less in a 142mm square area of the substrate main surface.

-   -   Polishing Liquid: cerium oxide (average grain size 1 μm)+water    -   Polishing Pad: soft polisher (suede type)

Then, a predetermined number of the glass substrates after the precisionpolishing were set in a double-side polishing machine and subjected tothe ultra-precision polishing process under the following polishingconditions. After the ultra-precision polishing process, the glasssubstrates were ultrasonically cleaned for removing polishing abrasivegrains adhering to the glass substrates. The polishing conditions suchas the processing pressure, the rotational speeds of upper and lowersurface plates, and the polishing time were properly adjusted. In thisultra-precision polishing process, the four corners tend to bepreferentially polished due to the substrate shape being square. Thepolishing conditions are set so that the flatness in the 142 mm squarearea of the substrate main surface does not exceed 0.3 μm while thesurface roughness of the substrate main surface becomes a predeterminedroughness of 0.4 nm or less.

In this manner, the glass substrates according to this invention weremanufactured.

-   -   Polishing Liquid: colloidal silica (average grain size 100        nm)+water    -   Polishing Pad: super-soft polisher (suede type)

The shape of the main surfaces of the glass substrates thus obtained wasmeasured by a wavelength-shift interferometer using a wavelengthmodulation laser. As a result, the shape of each glass substrate wassuch that the main surface where a thin film for a transfer pattern wasto be formed had a convex shape being relatively high at its center andrelatively low at its peripheral portion. The glass substrates having adifference of 40 nm or less upon fitting to the virtual reference mainsurface, i.e. passed products usable as mask blank substrates of thisinvention, were 95 out of 100 and thus could be manufactured with ahigher yield than that according to a conventional substrate selectionmethod (passed products were 80 out of 100).

Then, a back-surface antireflection layer, a light-shielding layer, anda front-surface antireflection layer were formed in this order as alight-shielding film on each of the glass substrates obtained asdescribed above. Specifically, using a Cr target as a sputtering targetand using a mixed gas of Ar, CO₂, N₂, and He (gas flow rate ratioAr:CO₂:N₂:He=24:29:12:35) as a sputtering gas, a CrOCN film was formedto a thickness of 39 nm as the back-surface antireflection layer bysetting the gas pressure to 0.2 Pa and the power of the DC power supplyto 1.7 kW. Then, using a Cr target as a sputtering target and using amixed gas of Ar, NO, and He (gas flow rate ratio Ar:NO:He=27:18:55) as asputtering gas, a CrON film was formed to a thickness of 17 nm as thelight-shielding layer by setting the gas pressure to 0.1 Pa and thepower of the DC power supply to 1.7 kW. Then, using a Cr target as asputtering target and using a mixed gas of Ar, CO₂, N₂, and He (gas flowrate ratio Ar:CO₂:N₂:He=21:37:11:31) as a sputtering gas, a CrOCN filmwas formed to a thickness of 14 nm as the front-surface antireflectionlayer by setting the gas pressure to 0.2 Pa and the power of the DCpower supply to 1.8 kW. The back-surface antireflection layer, thelight-shielding layer, and the front-surface antireflection layer formedunder these conditions had very low stress over the entirelight-shielding film and thus it was possible to suppress the change inshape of the substrate to minimum. In this manner, mask blanks weremanufactured.

By patterning the light-shielding film of each of the mask blanks thusobtained, photomasks (binary masks) were manufactured. The obtainedphotomasks were set in three exposure apparatuses having differentvacuum chuck structures and an inspection was conducted. The exposureapparatuses were of the type in which the chuck structure of a maskstage used, as chuck areas, portions of a main surface on two oppositeend face sides of a photomask, and a selection was made of two kinds ofexposure apparatuses in which portions with which the chuck areas of thephotomask were brought into contact were made of a low-elasticitymaterial, i.e. exposure apparatuses of a so-called hard chuck structure,and of one kind of exposure apparatus in which portions with which thechuck areas of the photomask were brought into contact were made of ahigh-elasticity material, i.e. exposure apparatus of a so-called softchuck structure. The flatness was derived by a wavelength-shiftinterferometer using a wavelength modulation laser. As a result, in anyof the exposure apparatuses, the flatness of the photomask afterchucking was 0.12 μm or less and it was possible to obtain excellenttransfer performance as a photomask of DRAM hp32 nm generation.

EXAMPLE 2

In Example 2, the shape of a virtual reference main surface of a virtualreference substrate to be aimed at was a curved surface shape in theform of a spherical surface having a radius of curvature r of 21,762,225mm and having a flatness of 0.2 μm in a 132 mm square area and glasssubstrates were manufactured by properly adjusting the polishingconditions in the same polishing processes as those in Example 1. Then,the shape of main surfaces of the glass substrates thus obtained wasmeasured by a wavelength-shift interferometer using a wavelengthmodulation laser. As a result, the shape of each glass substrate wassuch that the main surface where a thin film for a transfer pattern wasto be formed had a convex shape being relatively high at its center andrelatively low at its peripheral portion.

Then, fitting was carried out between the virtual reference main surfaceand each of the manufactured glass substrates. FIG. 8 is a contourdiagram showing the shape of the main surface measured by thewavelength-shift interferometer with respect to one of the manufacturedglass substrates. FIG. 9 shows the shapes of the main surface insections along diagonals (line XYR1-XYR1 and line XYR2-XYR2 in FIG. 8)of the glass substrate shown in FIG. 8. As a result of the measurement,the flatness in a 142 mm square area of this glass substrate was 0.19 μmand the flatness in a 132 mm square area thereof was 0.18 μm, thussatisfying the intended flatness of 0.2 μm or less. FIG. 10 is a contourdiagram showing the shape of the virtual reference main surface, forcarrying out fitting in its 132 mm square area, of the virtual referencesubstrate. FIG. 11 shows one sectional shape upon fitting the virtualreference main surface of FIG. 10 to the glass substrate of FIG. 8 inthe 132 mm square area. FIG. 12 shows differences between the mainsurface of the glass substrate and the ideal reference main surfaceshape upon performing the fitting in FIG. 11. With respect to thedifferences shown in FIG. 12, a portion where the height of the virtualreference main surface is higher than the height of the main surface ofthe glass substrate upon fitting is represented by a positive value,while, a portion where the height of the main surface of the glasssubstrate is higher upon fitting is represented by a negative value.

As seen from the results shown in FIG. 12, the fitting difference was0.0075 μm (7.5 nm) in positive value and −0.0067 μm (6.7 nm) in negativevalue and thus the results were very excellent. Even over the entire 132mm square area, the fitting difference was 0.011 μm (11 nm) at maximumand thus 40 nm or less and, therefore, the manufactured glass substratewas a passed product with high accuracy. Likewise, as a result offitting the virtual reference main surface to the other manufacturedglass substrates, the glass substrates having a difference of 40 nm orless upon fitting, i.e. passed products usable as mask blank substratesof this invention, were 90 out of 100 and thus could be manufacturedwith a higher yield than that according to a conventional substrateselection method (passed products were 80 out of 100).

Then, under the same film forming conditions as in Example 1, aback-surface antireflection layer, a light-shielding layer, and afront-surface antireflection layer were formed in this order as alight-shielding film on each of the glass substrates obtained asdescribed above, thereby manufacturing mask blanks. Further, bypatterning the light-shielding film of each of the mask blanks thusobtained, photomasks (binary masks) were manufactured. Like in Example1, the obtained photomasks were set in three kinds of exposureapparatuses having different vacuum chuck structures and an inspectionwas conducted. The flatness was derived by a wavelength-shiftinterferometer using a wavelength modulation laser. As a result, in anyof the exposure apparatuses, the flatness of the photomask afterchucking was 0.08 μm or less and it was possible to obtain sufficientlyexcellent transfer performance not only as a photomask of DRAM hp32 nmgeneration, but also as a photomask of DRAM hp22 nm generation.

EXAMPLE 3

A phase shift film and a laminated film, as a light-shielding film,composed of a back-surface antireflection layer, a light-shieldinglayer, and a front-surface antireflection layer were formed on each ofthe glass substrates manufactured in Example 1. Specifically, using amixed target of Mo and Si (at % ratio Mo:Si=10:90) as a sputteringtarget and using a mixed gas of Ar, N₂, and He (gas flow rate ratioAr:N₂:He=5:49:46) as a sputtering gas, a MoSiN film was formed to athickness of 69 nm as the phase shift film by setting the gas pressureto 0.3 Pa and the power of the DC power supply to 2.8 kW. Then, thesubstrates formed with the phase shift film were heat-treated (annealed)at 250° C. for 5 minutes.

Then, the light-shielding film comprising the back-surfaceantireflection layer, the light-shielding layer, and the front-surfaceantireflection layer was formed on the phase shift film. Specifically,first, using a Cr target as a sputtering target and using a mixed gas ofAr, CO₂, N₂, and He (gas flow rate ratio Ar:CO₂:N₂:He=22:39:6:33) as asputtering gas, a CrOCN film was formed to a thickness of 30 nm as theback-surface antireflection layer by setting the gas pressure to 0.2 Paand the power of the DC power supply to 1.7 kW. Then, using a Cr targetas a sputtering target and using a mixed gas of Ar and N₂ (gas flow rateratio Ar:N₂=83:17) as a sputtering gas, a CrN film was formed to athickness of 4 nm as the light-shielding layer by setting the gaspressure to 0.1 Pa and the power of the DC power supply to 1.7 kW. Then,using a Cr target as a sputtering target and using a mixed gas of Ar,CO₂, N₂, and He (gas flow rate ratio Ar:CO₂:N₂:He=21:37:11:31) as asputtering gas, a CrOCN film was formed to a thickness of 14 nm as thefront-surface antireflection layer by setting the gas pressure to 0.2 Paand the power of the DC power supply to 1.8 kW. The back-surfaceantireflection layer, the light-shielding layer, and the front-surfaceantireflection layer formed under these conditions had very low stressover the entire light-shielding film and the phase shift film also hadvery low stress, and thus it was possible to suppress the change inshape of the substrate to minimum.

By patterning the light-shielding film and the phase shift film of eachof mask blanks thus obtained, photomasks (phase shift masks) weremanufactured. Like in Example 1, the obtained photomasks were set inthree kinds of exposure apparatuses having different vacuum chuckstructures and an inspection was conducted. The flatness was derived bya wavelength-shift interferometer using a wavelength modulation laser.As a result, in any of the exposure apparatuses, the flatness of thephotomask after chucking was 0.12 μm or less and it was possible toobtain excellent transfer performance as a photomask of DRAM hp32 nmgeneration.

EXAMPLE 4

A phase shift film and a laminated film, as a light-shielding film,composed of a back-surface antireflection layer, a light-shieldinglayer, and a front-surface antireflection layer, all of which had thesame structures as in Example 3, were formed on each of the glasssubstrates manufactured in Example 2. By patterning the light-shieldingfilm and the phase shift film of each of mask blanks thus obtained,photomasks (phase shift masks) were manufactured. Like in Example 1, theobtained photomasks were set in three kinds of exposure apparatuseshaving different vacuum chuck structures and an inspection wasconducted. The flatness was derived by a wavelength-shift interferometerusing a wavelength modulation laser. As a result, in any of the exposureapparatuses, the flatness of the photomask after chucking was 0.08 μm orless and it was possible to obtain sufficiently excellent transferperformance not only as a photomask of DRAM hp32 nm generation, but alsoas a photomask of DRAM hp22 nm generation.

EXAMPLE 5

A MoSiON film (back-surface antireflection layer), a MoSi film(light-shielding layer), and a MoSiON film (front-surface antireflectionlayer) were formed as a light-shielding film on each of the glasssubstrates manufactured in Example 1. Specifically, using a mixed targetof Mo:Si=21:79 (at % ratio) as a sputtering target and using a mixed gasof Ar, O₂, N₂, and He (gas flow rate ratio Ar:O₂:N₂:He=5:4:49:42) as asputtering gas, a film made of molybdenum, silicon, oxygen, and nitrogen(MoSiON film: at % ratio of Mo and Si in the film was about 21:79) wasformed to a thickness of 7 nm by setting the sputtering gas pressure to0.2 Pa and the power of the DC power supply to 3.0 kW. Then, using thesame target and using Ar as a sputtering gas, a film made of molybdenumand silicon (MoSi film: at % ratio of Mo and Si in the film was about21:79) was formed to a thickness of 35 nm by setting the sputtering gaspressure to 0.1 Pa and the power of the DC power supply to 2.0 kW. Then,using a mixed target of Mo:Si=4:96 (at % ratio) as a sputtering targetand using a mixed gas of Ar, O₂, N₂, and He (gas flow rate ratioAr:O₂:N₂:He=5:4:49:42) as a sputtering gas, a film made of molybdenum,silicon, oxygen, and nitrogen (MoSiON film: at % ratio of Mo and Si inthe film was about 4:96) was formed to a thickness of 10 nm by settingthe sputtering gas pressure to 0.2 Pa and the power of the DC powersupply to 3.0 kW. The total thickness of the light-shielding film wasset to 52 nm. The back-surface antireflection layer, the light-shieldinglayer, and the front-surface antireflection layer formed under theseconditions had very low stress over the entire light-shielding film andthus it was possible to suppress the change in shape of the substrate tominimum.

By patterning the light-shielding film of each of mask blanks thusobtained, photomasks (binary masks) were manufactured. Like in Example1, the obtained photomasks were set in three kinds of exposureapparatuses having different vacuum chuck structures and an inspectionwas conducted. As a result, in any of the exposure apparatuses, theflatness of the photomask after chucking was 0.12 μm or less and it waspossible to obtain excellent transfer performance as a photomask of DRAMhp32 nm generation.

EXAMPLE 6

A light-shielding film in which a MoSiON film (back-surfaceantireflection layer), a MoSi film (light-shielding layer), and a MoSiONfilm (front-surface antireflection layer), all of which had the samestructures as in Example 5, were stacked in this order was formed oneach of the glass substrates manufactured in Example 2. By patterningthe light-shielding film of each of mask blanks thus obtained,photomasks (binary masks) were manufactured. Like in Example 1, theobtained photomasks were set in three kinds of exposure apparatuseshaving different vacuum chuck structures and an inspection wasconducted. The flatness was derived by a wavelength-shift interferometerusing a wavelength modulation laser. As a result, in any of the exposureapparatuses, the flatness of the photomask after chucking was 0.08 μm orless and it was possible to obtain sufficiently excellent transferperformance not only as a photomask of DRAM hp32 nm generation, but alsoas a photomask of DRAM hp22 nm generation.

EXAMPLE 7

Local processing by an MRF (Magneto-Rheological Finishing) processingmethod was applied to the main surfaces of the glass substrates havingbeen subjected to the ultra-precision polishing process and theultrasonic cleaning in Example 1. First, the flatness of the mainsurface of the glass substrate was measured by a wavelength-shiftinterferometer using a wavelength modulation laser (measurement area:142 mm square area having its center at the center of the substrate).Then, based on measured values, it was verified whether the flatness ofthe substrate main surface in the 142 mm square area was 0.3 μm or less.If the flatness exceeded 0.3 μm, a portion having a height exceeding 0.3μm as seen from the lowest portion was specified as a portion wherelocal processing was necessary, and the required processing amount wascalculated. Then, based on the measured values of the substrate mainsurface, the reference curved surface of the virtual reference substratewas fitted to the 132 mm square area of the substrate main surface. Inthis case, the fitting was performed so that the reference curvedsurface was not located at a height above the predetermined maximumallowable fitting difference (40 nm) with respect to the substrate mainsurface in the 132 mm square area. Then, a portion of the substrate mainsurface located above the predetermined maximum allowable fittingdifference (40 nm) with respect to the reference curved surface fittedwas specified as a portion where local processing was necessary, and therequired processing amount was calculated. At this stage, the substratefor which it was judged that no local processing was necessary was apassed product usable as a mask blank substrate of this invention.

Then, local processing by the MRF processing method was applied to theglass substrate for which the local processing was judged necessary anda portion for the local processing was specified. The MRF processingmethod is a method of locally polishing a substrate by bringingpolishing abrasive grains contained in a magnetic fluid into contactwith the substrate by magnetic field assist and controlling the staytime of the polishing abrasive grains at a contact portion. In thispolishing, as the convex degree of a convex portion increases, the staytime of the polishing abrasive grains at a contact portion is set to belonger. Conversely, as the convex degree of a convex portion decreases,the stay time of the polishing abrasive grains at a contact portion isset to be shorter.

FIGS. 13A and 13B are schematic diagrams for explaining a processingstate by the MRF processing method, wherein FIG. 13A is a frontsectional view and FIG. 13B is a side sectional view. In the figures,according to the MRF processing method, a mask blank substrate 1 being aworkpiece is locally polished by bringing polishing abrasive grains (notillustrated) contained in a magnetic fluid 41 containing iron (notillustrated) into contact with the mask blank substrate 1 at high speedby magnetic field assistance and controlling the stay time of thepolishing abrasive grains at a contact portion. That is, a mixed liquid(magnetic polishing slurry 4) of the magnetic fluid 41 and a polishingslurry 42 is loaded on a rotatably supported disk-shaped electromagnet 6so that a radial end of the magnetic polishing slurry 4 is used as apolishing spot 5 for local processing, and a convex portion 13 to beremoved is brought into contact with the polishing spot 5. With thisconfiguration, the magnetic polishing slurry 4 flows along a disk-shapedmagnetic field in a substantially two-layer state where the polishingslurry 42 is distributed much on the substrate 1 side and the magneticfluid 1 is distributed much on the electromagnet 6 side. By using partof this state as the polishing spot 5 for carrying out local polishingand bringing it into contact with a surface of the substrate 1, theconvex portion 13 is locally polished and controlled to a flatness ofseveral tens of nanometers.

In this MRF processing method, as different from a conventionalpolishing method, the polishing spot 5 is constantly flowing and thusthere is no degradation of processing accuracy due to abrasion of aprocessing tool or a change in shape thereof and, further, it is notnecessary to press the substrate 1 under high load and therefore thereis an advantage in that cracks or hidden cracks in a surfacedisplacement layer are small in number. Further, in the MRF method, theremoval amount can be easily adjusted by controlling the moving speed ofthe substrate 1 according to a machining allowance (required processingamount) set per predetermined area, when moving the substrate 1 whilekeeping the polishing spot 5 in contact with the substrate 1.

As the polishing slurry 42 mixed into the magnetic fluid 41, use is madeof a slurry in which fine polishing grains are dispersed in a liquid.The polishing grains are, for example, silicon carbide, aluminum oxide,diamond, cerium oxide, zirconium oxide, manganese oxide, colloidalsilica, or the like and are properly selected according to a material ofa workpiece, the processing surface roughness thereof, and so on. Thepolishing grains are dispersed into a liquid such as water, an acidsolution, or an alkaline solution to form the polishing slurry 42 whichis then mixed into the magnetic fluid 41.

With respect to a portion where local polishing by the MRF processingmethod was judged necessary as a result of carrying out fitting betweena main surface of the mask blank substrate 1 and the virtual referencemain surface, the local polishing was carried out by a calculatedrequired processing amount. Then, since the main surface having beensubjected to the local polishing was roughened, double-side polishingwas performed only for a short time using a double-side polishingmachine. The double-side polishing was performed under the followingpolishing conditions. The polishing conditions such as the processingpressure, the rotational speeds of upper and lower surface plates, andthe polishing time were properly adjusted.

-   -   Polishing Liquid: colloidal silica (average grain size 70        nm)+alkaline aqueous solution (NaOH, pH11)    -   Polishing Pad: super-soft polisher (suede type)

As a result, the shape of each glass substrate was such that the mainsurface where a thin film for a transfer pattern was to be formed had aconvex shape being relatively high at its center and relatively low atits peripheral portion. The glass substrates having a difference of 40nm or less upon fitting to the virtual reference main surface, i.e.passed products usable as mask blank substrates of this invention, were100 out of 100 and thus could be manufactured with an extremely highyield.

Then, a light-shielding film was formed on each of the glass substratesthus obtained in the same manner as in Example 1, thereby manufacturingmask blanks. Further, by patterning the light-shielding film of each ofthe mask blanks thus obtained, photomasks (binary masks) weremanufactured. Like in Example 1, the obtained photomasks were set inthree kinds of exposure apparatuses having different vacuum chuckstructures and an inspection was conducted. The flatness was derived bya wavelength-shift interferometer using a wavelength modulation laser.As a result, in any of the exposure apparatuses, the flatness of thephotomask after chucking was 0.12 μm or less and it was possible toobtain excellent transfer performance as a photomask of DRAM hp32 nmgeneration.

EXAMPLE 8

Like in Example 7, local processing by the MRF processing method wasapplied to the main surfaces of the glass substrates having beensubjected to the ultra-precision polishing process and the ultrasoniccleaning in Example 2. Herein, the local processing was carried out sothat the flatness of the substrate main surface in the 142 mm squarearea became 0.3 μm or less and, further, the flatness of the substratemain surface in the 132 mm square area became 0.2 μm or less. As aresult, the shape of each glass substrate was such that the main surfacewhere a thin film for a transfer pattern was to be formed had a convexshape being relatively high at its center and relatively low at itsperipheral portion. The glass substrates having a difference of 40 nm orless upon fitting to the virtual reference main surface, i.e. passedproducts usable as mask blank substrates of this invention, were 100 outof 100 and thus could be manufactured with an extremely high yield.

Then, a light-shielding film was formed on each of the glass substratesthus obtained in the same manner as in Example 1, thereby manufacturingmask blanks. Further, by patterning the light-shielding film of each ofthe mask blanks thus obtained, photomasks (binary masks) weremanufactured. Like in Example 1, the obtained photomasks were set inthree kinds of exposure apparatuses having different vacuum chuckstructures and an inspection was conducted. The flatness was derived bya wavelength-shift interferometer using a wavelength modulation laser.As a result, in any of the exposure apparatuses, the flatness of thephotomask after chucking was 0.08 μm or less and it was possible toobtain sufficiently excellent transfer performance not only as aphotomask of DRAM hp32 nm generation, but also as a photomask of DRAMhp22 nm generation.

As described above, according to this invention, a mask blank substrateis such that a main surface where a thin film for a transfer pattern isto be formed has a convex shape being relatively high at its center andrelatively low at its peripheral portion and has a height differencewithin ±40 nm with respect to a predetermined virtual reference mainsurface of a virtual reference substrate. Therefore, it is not necessaryto simulate the flatness of the mask substrate after chucking it on amask stage of an exposure apparatus and, further, it is possible torealize a desired flatness thereof after chucking regardless of thechuck structure of an exposure apparatus.

This invention is not limited to the above-mentioned embodiment and canbe carried out by appropriately changing it. For example, the materials,sizes, processing sequences, and so on in the above-mentioned embodimentare only examples and this invention can be carried out by changing themin various ways within a range capable of exhibiting the effect of thisinvention. Other than that, this invention can be carried out in variousways within a range not departing from the object of this invention.

What is claimed is:
 1. A mask blank substrate for a photomask to bechucked on a mask stage of an exposure apparatus, wherein; with respectto the mask blank substrate, a difference upon fitting, to a mainsurface of the mask blank substrate, a virtual reference main surface ofa virtual reference substrate in a 132 mm square area including itscentral portion is 40 nm or less, the main surface being on a side wherea thin film for a transfer pattern is to be formed, and the virtualreference main surface has a convex shape being relatively high at itscentral portion and relatively low at its peripheral portion and has ashape specified by a complete spherical surface in the 132 mm squarearea.
 2. The mask blank substrate according to claim 1, wherein aflatness in the 132 mm square area of the virtual reference main surfaceis 0.3 μm or less.
 3. The mask blank substrate according to claim 2,wherein the virtual reference main surface has a radius of curvature of14,500,000 mm or more.
 4. The mask blank substrate according to claim 1,wherein the main surface of the mask blank substrate has a flatness of0.3 μm or less in a 142 mm square area including its central portion. 5.A mask blank, comprising: the mask blank substrate according to claim 1,and a thin film formed on the main surface of the mask blank substrate.6. A mask blank for a photomask to be chucked on a mask stage of anexposure apparatus, wherein; with respect to the mask blank, adifference upon fitting, to a main surface of the mask blank, a virtualreference main surface of a virtual reference substrate in a 132 mmsquare area including its central portion is 40 nm or less, the mainsurface being on a side where a thin film for a transfer pattern is tobe formed, and the virtual reference main surface has a convex shapebeing relatively high at its central portion and relatively low at itsperipheral portion and has a shape specified by a complete sphericalsurface in a 132 mm square area.
 7. The mask blank according to claim 6,wherein a flatness in the 132 mm square area of the virtual referencemain surface is 0.3 μm or less.
 8. The mask blank according to claim 7,wherein the virtual reference main surface has a radius of curvature of14,500,000 mm or more.
 9. The mask blank according to claim 6, whereinthe main surface of the mask blank has a flatness of 0.3 μm or less in a142 mm square area including its central portion.
 10. The mask blankaccording to claim 1 or 6, wherein the thin film is a light-shieldingfilm made of a material containing chromium or a material containingmolybdenum silicide.
 11. A photomask having a transfer pattern formed bythe thin film of the mask blank according to claim 5 or
 6. 12. A maskblank substrate manufacturing method, comprising the steps of: polishinga main surface, on a side where a thin film is to be formed, of asubstrate; measuring a surface shape of the main surface of thesubstrate in a 132 mm square area thereof, performing fitting betweenthe measured surface shape of the substrate and a virtual reference mainsurface of a virtual reference substrate, and selecting as a mask blanksubstrate the substrate if a difference between the measured surfaceshape and the virtual reference main surface is 40 nm or less, whereinthe virtual reference main surface has a convex shape being relativelyhigh at its central portion and relatively low at its peripheral portionand has a shape specified by a complete spherical surface in a 132 mmsquare area.
 13. A mask blank substrate manufacturing method accordingto claim 12, wherein a flatness in the 132 mm square area of the virtualreference main surface is 0.3 μm or less.
 14. A mask blank substratemanufacturing method according to claim 13, wherein the virtualreference main surface has a radius of curvature of 14,500,000 mm ormore.
 15. A mask blank substrate manufacturing method according to claim12, wherein the main surface of the substrate has a flatness of 0.3 μmor less in a 142 mm square area including its central portion.
 16. Amask blank manufacturing method, comprising a step of: forming a thinfilm on the main surface of the substrate obtained by the methodaccording to claim
 12. 17. A photomask manufacturing method, comprisinga step of: forming a transfer pattern in the thin film of the mask blankobtained by the method according to claim
 16. 18. A semiconductor devicemanufacturing method using a photomask obtained by the method accordingto claim 17 to transfer a transfer pattern of the photomask onto asemiconductor substrate by lithography.